/Engine/Source/Runtime/UMG/Public/Blueprint/UserWidget.h
Called when the user cancels the drag operation, typically when they simply release the mouse button after beginning a drag operation, but failing to complete the drag.
void OnDragCancelled ( const [FPointerEvent](API\Runtime\SlateCore\Input\FPointerEvent) & PointerEvent, [UDragDropOperation](API\Runtime\UMG\Blueprint\UDragDropOperation) * Operation )